In a semiconductor fabrication process such as an etching process or a chemical vapor deposition (CVD) process, fluorine compounds such as hydrofluorocarbon (e.g., CHF3) or perfluorocompound (e.g., CF4, C2F6, C3F8, C4F8, C5F8, C4F6, SF6, and NF3) have been used in a system. In some cases, CO, NH3 or O2 may be used in a semiconductor fabrication apparatus. An exhaust gas discharged from a semiconductor fabrication apparatus which uses fluorine compounds, CO or NH3 includes harmful components such as CO, NH3, SiF4, F2, COF2, C5F8, C4F6, or NF3. The exhaust gas also includes fluorine compounds which are not harmful but affect the global warming. Therefore, when an exhaust gas is discharged from a semiconductor fabrication apparatus using fluorine compounds or CO into an environmental atmosphere, it is necessary to detoxify a harmful gas included in the exhaust gas and also to decompose a gas which causes the global warming.
In a conventional method of treating a harmful gas  (SiF4, F2, COF2, C5F8, C4F6, or NH3) included in an exhaust gas, harmful components are adsorbed by an adsorbent such as a synthetic zeolite. However, in this conventional method, CO or perfluorocompound (PFC) cannot be removed from the exhaust gas. Further, the adsorbent needs to be replaced with a new one periodically, resulting in increased running cost.
A wet scrubber has been used to scrub an exhaust gas for removing a water-soluble gas and a hydrolytic gas such as SiF4, F2 or NH3 from the exhaust gas. However, such a wet scrubber cannot remove gases that are not water-soluble, such as CO and PFC.
In a conventional method of removing CO from an exhaust gas, CO is oxidized into CO2 with an oxidation catalyst and O2. However, in this conventional method, PFC cannot be removed by the decomposition. If an acid gas, such as SiF4 or F2, or PFC is brought into contact with the oxidation catalyst, then the oxidation catalyst is poisoned by the acid gas or PFC to lower its CO oxidation capability. Therefore, the oxidation catalyst needs to be replaced with a new one periodically.
When the aforementioned conventional methods of adsorption, wet scrubbing, and oxidation are combined with each other, it is possible to treat a harmful gas and CO in an exhaust gas simultaneously. However, these treatments need to have components replaced periodically, resulting in increased running cost. Further, PFC still cannot be removed from the exhaust gas with the above combined method.
There has been proposed a method of removing PFC from an exhaust gas with various types of catalysts for decomposing PFC. However, if the catalyst is deteriorated, then harmful components such as CO, C5F8, and C4F6 may be discharged into the environmental atmosphere immediately after deterioration of the catalyst. There has also been proposed a method of treating PFC by combustion. However, NOx or CO may be produced as a by-product gas, depending on the combustion conditions. Since this method requires a fuel such as H2, natural gas (city gas), or propane gas, it is necessary to provide equipment for supplying the fuel. Further, a complicated process is required to manage the operation. There has also been proposed a method of decomposing PFC by heating oxidation. However, in order to decompose PFC (e.g., CF4) that is unlikely to otherwise be decomposed, an exhaust gas should be heated to a high temperature of 1400° C. or higher. In such a case, loads applied on materials and a heater in the system become considerably large.
There has been proposed a method in which NH3, a lower saturated hydrocarbon gas, or a lower unsaturated hydrocarbon gas is added to an exhaust gas, and PFC is decomposed by heating oxidation without free O2 gas. However, there has not been proposed a method of heating oxidative decomposing PFC with the coexistence of O2. Further, there has also been proposed a method of decomposing PFC with a plasma in the presence of water (H2O). However, when PFC is decomposed, a harmful gas such as CO or HF is produced and thermal NOx is also produced. Therefore, it is necessary to provide a separate exhaust gas treatment apparatus for treating the harmful gas and the thermal NOx.